Breast cancer is the most frequent cancer and the second cause of cancer death in women in North America, with available therapies showing limitations. Tamoxifen, a widely used antiestrogen, has mixed agonist-antagonist activities, and steroidal antiestrogens like ICI 164,384 have limited oral bioavailability. We synthesized a new nonsteroidal antiestrogen, (S)-(+)-4-[7-(2,2-Dimethyl-1-oxopropoxy)-4-methyl-2-[4-[2-(1-piperidinyl) ethoxy]phenyl]-2H-1-benzopyran-3-yl] phenyl 2,2-Dimethylpropanoate (EM-800, (S)-1), via resolution of racemic chromene 6 and prodrug derivatization of the active enantiomer (S)-6 (EM-652). We compared the biological properties of enantiomers (R)-1/(R)-6 and (S)-1/(S)-6. Results showed (S)-6/(S)-1 had much higher estrogen receptor affinity (e.g., (S)-6 had Ki 0.047 nM in human breast cancer cytosol, relative binding affinity (RBA) 291 vs estradiol's 100) and potency in inhibiting estradiol-stimulated proliferation of T-47D and ZR-75-1 human breast cancer cells (e.g., (S)-1 had IC50 0.14 nM vs (R)-1's 36.2 nM in T-47D cells, 260-fold higher potency). In vivo, (S)-1 was at least 30 times more potent than (R)-1 in inhibiting estrone-stimulated uterine weight in ovariectomized mice, and acted as a pure estrogen antagonist in mammary gland, endometrium, and hypothalamopituitary feedback without agonist activity. Additionally, EM-800 increased bone mineral density and decreased serum cholesterol and triglycerides in rats. EM-800 [(S)-1] is the most potent antiestrogen described so far, with high specificity and oral activity. The availability of such a potent and specific antiestrogen could lead to significant improvement in the treatment of estrogen-sensitive breast and uterine cancer as well as nonmalignant estrogen-sensitive diseases.